Photopolymerization is a process of formation of cross-linked polymers from monomers or oligomers under exposure of light. Photopolymerization is also commonly recognized as a green and sustainable technology characterized by low electrical power input and energy requirements. The phase transition from liquid to solid in the light-induced polymerization reactions forms the basis of numerous traditional industrial applications of photopolymers in coatings, inks, adhesives, and optoelectronics. Photopolymerization technology is now expanded into a number of new emerging applications such as additive manufacturing (stereolithography and polyject) and nanotechnology due to their unique benefits of low temperature operation and no volatile organic compounds release (1. Photoresponsive Polymers; Krongauz, V., Trifunac, A., Eds.; Chapman and Hall: New York, N.Y., USA, 1994; 2. Belfied, K. D.; Crivello, J. V. Photoinitiated Polymerization; ACS Symp. Ser. 847; American Chemical Society: Washington, D.C., USA, 2003; 3. Green, W. A. Industrial Photoinitiators; CRC Press: Boca Raton, Fla., USA, 2010; 4. Photochemistry and Photophysics of Polymer Materials; Allen, N. S., Ed.; Wiley: Hoboken, N.J., USA, 2010).
Free radical photopolymerization (FRP) is undoubtedly the most popular photopolymerization technique, which involves a polymerizable radical matrix and a photoinitiating system (PIS). The PIS contains at least a photoinitiator (PI) and/or a photosensitizer (PS) to absorb the light (Fouassier, J. P.; Lalevée, J. Photoinitiators for Polymer Synthesis: Scope, Reactivity and Efficiency; Wiley VCH: Weinheim, Germany, 2012). FRP process is based on the application of a photoinitiating system, which converts absorbed light radiation into chemical energy in form of initiating species, such as free radicals, to induce polymerization of monomers. Nowadays, the FRP has made significant inroads in replacing traditional thermally cured and solvent-based technologies in large scale and high throughout manufacturing applications due to its high rates of polymerization and elimination of the use of volatile organic solvents. Along with many examples of commercial success of FRP in industry applications, the use of PIs in the polymerization processes still remains some safety and engineering issues. The discovery of contamination of baby milk with Isopropyl Thioxanthone (ITX) in 2005 caused a huge stir in the European food packaging market, where ITX was used as a photoinitiator in UV curable printing inks on the outer surface of liquid milk cartons. In 2009, the German authorities reported a migration of 4-Methyl Benzophenone (4-MBP) to muesli above the Specific Migration Limit (SML). Both PIs (ITX and 4-MBP) were able to migrate into the food due to their very small molecular weight. Furthermore, the residual PIs in the resulting polymer resins often lead to decrease of the mechanical properties and thermal stability of the resulting materials. Therefore, a new strategy to decrease PIs migration together with enhanced mechanical and thermal properties for photopolymerization is greatly anticipated (Oku J. Impact Properties of Acrylic Denture Base Resin. Part 2 Effects of Temperature and Residual Monomer on Impact Characteristics. Dent Mater J 1989; 8:186-193).
Hou et al. (Hou et al., Adv. Mater. Interfaces 2014, 1, 1400385) report the preparation of F2-POSS-(SH)4-TX-EDB and its use in initiating photopolymerization. However, each molecule of said compound comprises four thiol groups, two fluorinated carbon chains, the photoinitiator moiety TX and the co-initiator moiety EDB. These functional groups may course sterical issues (specifically after curing of the photopolymerizable polymer) that massively influence the chemical and physical properties of the prepared resin. Moreover, thiol groups and fluorinated carbon chains may provide safety drawbacks if the prepared resin is exposed to sensitive products, such as food, drinks, cosmetics etc.
EP 2370449 B1 discloses a photoactive moiety and an amine functionality, preferably a tertiary amino group, bonded to a polyhedral oligomeric silsesquioxane. Due to the branched amine functionality also these compounds, as described above, may course sterical issues (specifically after curing of the photopolymerizable polymer) that massively influence the chemical and physical properties of the prepared resin.
Hence, there is need in the art for photoinitiator compounds and resins/polymers prepared from said photoinitiator compounds that provide a safe use (also in sensitive products) and that demonstrate improved chemical and physical properties.